I have not tried this, but I would think that it would be worth trying and should be fine to use during pregnancy. These suggestions come from a book "Over 100 Helpful Household Hints For Vinegar" in the chapter titled Beauty & Relaxation
"Cleansers & toners
Use a mixture of 1/2 vinegar, 1/2 water to clean your face. Then rinse with vinegar diluted with water, & let face air-dry to seal in moisture.
Problem Skin
Oily skin can be controlled with mixture of 1/2 apple cider vinegar and 1/2 cool water. The mixture works as an astringent. You can also freeze solution into ice cubes and use as a cooling facial treatment on a hot days.
Make a paste of honey, wheat flour, and vinegar, then use it to lightly cover a new outbreak of pimples. Keep paste on overnight, and rinse off in the morning. This should accelerate healing.
Acne Treatment
Use a clean travel-size bottle to mix 1 tsp vinegar and 10 tsp of water. Clean your face as usual in the morning, then carry this bottle and a few cotton balls with you so you can dab acne spots several times during the day. This solution shouldn't dry out skin, the vinegar should help return skin to natural ph balance & may help prevent future acne outbreaks. Discontinue if irritation worsens."
I have had success with other tips that I have used in this book. Good Luck